Coal fly ash is produced by coal-fired electric and steam generating plants and other industrial facilities. Typically, coal is pulverized and blown with air into the boiler's combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses.
The American Coal Ash Association reports that 70,150,000 tons of fly ash was produced in 2003 and that 27,136,524 were beneficially utilized, while the remainder was disposed in lagoons and landfills. The most prevalent utilization application for fly ash (12,265,169 tons) was in concrete as pozzolan. Pozzolans are siliceous or siliceous and aluminous materials, which in a finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperatures to produce cementitious compounds.
A substantial portion of fly ash particles are reactive glass, which will combine with alkali hydrates, especially calcium hydroxide, that are formed as cement hydrates in plastic concrete. This chemical reaction is referred to as a pozzolanic reaction and the result of this reaction is a stable cementious bond, similar to the bond that results through the hydration products of cement, particularly, calcium-silica-hydrate (sometimes referred to as tobermorite gel). The cementitious bonds produced through the pozzolanic reaction of fly ash effect an increase in the strength and durability of the concrete. The strength-producing characteristics of fly ash allow for a lower amount of cement than would otherwise be needed. The value of fly ash as pozzolan is generally related to the cost of the portion of cement that is replaced by the fly ash.
Due to strategies implemented by electric utilities to meet lower emission limits, whether self-imposed or instituted by government regulations, such as the Clean Air Act Amendments of 1990, coal-burning operations have changed and will continue to change over the next decades. Generally speaking, these changes in coal-burning operations are intended to reduce the emissions of particulate matter or polluting gases, such as sulfur and nitrogen oxides. Individually these pollution control methods are designed to limit emissions of, among other things, (1) dust and/or very fine particulate matter, which are associated with increased rates of hospital admissions, respiratory disease and mortality, especially for mortality due to respiratory and cardiovascular disease for infants and the elderly, (2) fumes of sulfur oxides (SOx), which are directly related to the concentrations and quantities of acid rain, (3) fumes of nitrogen oxides (NOx), which are precursors for ground level ozone, and (4) mercury and other heavy metals, especially those that are considered to be persistent bioaccumulative toxins.
It is estimated that by the year 2010, these new pollution control strategies will result in 6,400 fewer premature deaths, and also create a savings of nearly $40 billion dollars in health care costs. These numbers increase to 12,000 fewer deaths and $93 billion dollars in health care savings by the year 2020.
Unfortunately, some of the unintended consequences of these pollution control methods have negatively impacted the utilization of the coal fly ash, especially as pozzolan in concrete. According to the ACAA less than 17.5% of the fly ash produced in 2003 was used as pozzolan. However, in many parts of the U.S., the demand for fly ash as pozzolan is significantly higher than the local supply of pozzolan-grade fly ash. A major reason for this shortage of “quality” fly ash is caused by changes in the characteristics of fly ash produced in the U.S. due to changes made to coal-burning operations and to the increased use of post-combustion pollution control techniques implemented by electric utilities to meet lower emission limits.
According to the U.S. Department of Transportation's Federal Highway Administration, changes in boiler operations and/or air emissions control systems at power plants will continue to alter the quality of fly ash produced. Factors that impact ash quality in this way include: a reduction in the pozzolanic reactivity; the presence in the fly ash of excessive unburned carbon; and, chemical residuals from post-combustion emission control.
It would be desirable to beneficially alter the characteristics of coal fly ash, especially the fly ashes that have been negatively affected by the aforementioned pollution control methods. Generically, such processes are called beneficiation processes; specifically, it would be desirable to have a thermal beneficiation process that is especially designed to alter these particular characteristics.
There are three major issues which affect the value and utility of fly ash used as pozzolan in concrete: the pozzolanic, strength-producing characteristics; the air-entraining characteristics; and, the presence of foreign residual chemicals.
Pozzolanic reactivity is typically measured by the compressive strength ratio between plain portland cement and pozzolan-containing concretes. U.S. ASTM C 618 as presently written specifies minimum “strength activity index” performance properties, which compares the compressive strength of a control concrete specimen made of plain portland cement to a pozzolan-enhanced cement concrete specimen. Unfortunately, since complete strength activity index testing takes at least one month, there is a substantial lag time before definitive quantification of pozzolanic reactivity can be determined. Fortunately, there is a very good correlation between the specific surface area of the fly ash glass and pozzolanic reactivity with both cement and lime. Therefore, quick calculation of the specific surface area of the fly ash glass can be used to infer pozzolanic reactivity.
There are several other tests and techniques to infer pozzolanic reactivity. Unfortunately, the correlations between these single-variable test results and pozzolanic reactivity are typically very poor. For example, since pozzolanic reactivity in concrete is mostly related to the reaction between the glass in fly ash and alkalis present in the cement paste matrix, chemical requirements for the fly ash are often used to infer the quantity the reactive glass and, consequently, to classify grades of fly ash and/or predict pozzolanic strength activity. Specifically, pozzolanic reactivity is mostly related to the reaction between the reactive silica glass and calcium hydroxide producing calcium silicate hydrate. The alumina in the pozzolan will also react in the cement paste matrix, producing calcium aluminate hydrate, ettringite, gehlenite, and calcium monosulphoaluminate hydrate. ASTM and some other standards associations have also included the iron oxide content as a major requirement.
However, there is a poor correlation between the sum of these oxides (i.e., silica, alumina, and iron) and compressive strength. This may be partly due to the presence of varying amounts of nonreactive, crystallized phases of silica and alumina (quartz and mullite) and/or inconsistent, and sometimes counterproductive, contribution of iron oxide to strength development. Consequently, using the sum of these oxides is not considered to be a good technique to infer the strength activity of a fly ash.
Therefore, in summary, pozzolanic reactivity can best be quantified through a “strength activity index,” but determination of specific surface area for the fly ash glass will approximate changes in pozzolanic reactivity. Increasing the glass to crystalline ratio of fly ash will increase the pozzolanic reactivity and, consequently, the value of the fly ash.
Due to the lower combustion temperatures necessary to reduce the formation of thermal NOx, commercial operation of low-NOx burners produce coal ash that has not been exposed to the high-temperature operating conditions employed before low-NOx combustion techniques were implemented. The operating temperatures employed for low-NOx combustion may be below the melting point of individual constituents of the mineral matter contained in the coal being burned, especially for the higher ash fusion bituminous coals. Consequently, the amount of mineral mater that has become molten and then air-cooled, thereby forming reactive glass, may be reduced significantly due to low-NOx combustion of coal.
There is very little published literature about the reduced pozzolanic reactivity of fly ashes produced through low-NOx burning of pulverized coal. However, according to U.S. Department of Energy publications (see, “Technical Overview of Recent and Ongoing Developments” by William Ellison, Ellison Consultants, Monrovia, Md., and Fred E. Preik, PWR Solutions, Valencia, Pa., application of low-NOx combustion “ . . . is seen to hinder high-value fly ash utilization in two ways: (1) much publicized increase in unburned carbon content of fly ash that inhibits commercial use in air-entrained concrete; and, (2) little mentioned, or understood, impairment of ash pozzolanic properties, caused by the greatly reduced fuel firing temperature . . . ”
Decreasing the glass to crystalline ratio of fly ash will decrease the pozzolanic reactivity and consequently the value of fly ash. The pozzolanic reaction of fly ash glass is also known to increase the long-term durability of concrete. When fly ash is used as pozzolan in concrete, the density of the concrete paste matrix may increase significantly and, therefore, the durability of the concrete may be significantly greater than with ordinary portland cement alone. Increasing the glass to crystalline ratio of fly ash will increase the pozzolanic reactivity, making the concrete more impervious and, therefore, more durable.
A second major issue affecting the utilization and value of fly ash—the air-entraining characteristics of the fly ash—is also related to the impact of fly ash on concrete durability. Particularly, one aspect of concrete durability, namely, freeze-thaw durability, may be negatively impacted by the presence of fly ash, especially by the presence of unburned carbon that remains in fly ash following the coal-burning operation.
There are many factors that can affect the durability of concrete to cycles of freezing and thawing; however, the single greatest impact on freeze thaw durability derives from the presence and uniform distribution of air voids in the hardened cement paste matrix with optimum spacing and size.
When hardened concrete begins to freeze, residual water inside concrete will also freeze; when water freezes its volume increases 9%. The expanding ice forces water into the unfrozen regions of the cement binder. This movement of water creates large hydraulic pressures and generates tensile stress. Although concrete has excellent strength in compression, its tensile strength is less than 10% of the compressive strength. When the tensile stress exceeds the tensile strength of the concrete, cracking and deterioration occurs. A network of air voids with the proper spacing and size distribution in the hardened cement paste matrix allows the water to expand and migrate deeper into the concrete, reducing the hydraulic pressure and tensile stress in the concrete.
Air is naturally entrapped in the cement paste of plastic concrete through the folding and shearing action of the mixing process. However, the entrapped air voids are large and not stable in concrete without the use of surface active agents, commonly called surfactants. Surfactants can be used in the production of concrete to reduce the surface tension of water. Consequently, large air voids will divide into smaller, more stable air voids. Air entraining agents (AEAs) are commonly used as surfactants in the production of concrete designed to increase freeze-thaw durability.
Residual unburned carbon in fly ash can have a high adsorptive capacity for AEAs. More specifically, there are certain active sites on the carbon surface, which are typically nonpolar, that preferentially adsorb AEAs from the aqueous phase. The rate of AEA adsorption will vary according to the type, amount, and/or level of activated carbon surface area, requiring a varying increase in AEA dosage to maintain the desired entrained air void system or else resulting in an inconsistent level of entrained air in the hardened concrete, which will ultimately affect the strength and/or durability of the concrete by degrading the air void system. There is also an increased risk of over-dosing the AEA and creating an elevated entrained air content, which would negatively impact the strength of the hardened concrete.
The use of fly ash as pozzolan is typically controlled by specifications that effectually limit the amount of unburned carbon that can remain in fly ash used as pozzolan. Most specifications prescribe a maximum limit for the Loss On Ignition (LOI) of fly ash used as pozzolan in concrete. LOI is a percent-by-weight measure of the residual combustible material, primarily carbon, in the fly ash. The strength-producing characteristics of a fly ash are relatively unaffected by LOI levels up to and above 12%; therefore, the low maximum limits prescribed by most of the controlling specifications for fly ash as pozzolan in the U.S. are not necessary to assure the strength-producing characteristics. Instead, the intent of these low maximum LOI limits is to assure adequate air-entraining characteristics for pozzolan-grade fly ash used to produce air-entrained concrete. The concrete industry also references specific LOI values for fly ash to predict and/or monitor the air-entraining characteristics of the various fly ashes available in the marketplace and there is a general perception that lower LOI levels equate to higher quality.
There are several processes in commercial use that aim to significantly reduce the LOI of moderate to high LOI fly ashes—to a level below 3% by weight, specifically triboelectric separation and carbon combustion. It should be noted that carbon makes up most of the measured LOI (to within about 10%); however, as previously discussed, it is the adsorptive capacity of the fly ash, especially the active carbon sites, for air entraining agents and not the LOI per se, that impacts the marketability of the fly ash. At this time, there is a growing realization that lower LOI fly ashes do not assure superior, or even adequate, air-entraining characteristics for many fly ashes.
Therefore, regardless of the specific reduction of LOI through combustion, it would be desirable to beneficially alter the air-entraining characteristics of the processed fly ash by reducing the overall adsorptive capacity of the fly ash.
A third major issue affecting the utilization and marketability of fly ash also derives from operational changes in the coal-burning process, specifically the presence of residual chemicals and/or particulate matter deposited in or adsorbed on the coal fly ash during the coal-burning operation and/or subsequent flue gas treatment processes, especially those processes intended to reduce air pollution. These changes in coal-burning operations are intended to reduce the emissions of particulate matter; polluting gases, such as sulfur and nitrogen oxides; and heavy metals, such as mercury, or other toxic emissions, especially those considered to be persistent bioaccumulative toxins.
There are several different techniques for the reduction of each of the above pollutants and coal-burning operations often utilize a combination of some or all of these pollution control techniques in order to meet the targeted emission levels. One example of these pollution control techniques, namely, flue gas conditioning, is used to enhance precipitator performance. This technique deliberately deposits foreign chemicals, particularly ammonia, sulfur, and other proprietary chemicals, on the coal fly ash. This technique actually “conditions” the fly ash by coating the particles with these chemicals, changing the surface conductivity and, therefore, the resistivity of the fly ash. These chemicals may also create a space-charge effect and improve the cohesiveness of the fly ash particles.
Injecting these chemicals in the hot flue gases will improve the efficiency of electrostatic precipitators and, therefore, the collection rates for the coal fly ash. However, the collected fly ash will have increased levels of ammonia, sulfur oxides, and/or other residual chemicals which are known to negatively impact the marketability of fly ash as pozzolan at high concentration levels.
Additional pollution control techniques include, but are not limited to, fuel switching and/or blending, the use of low-NOx burners, flue gas treatment to enhance the performance of NOx scrubbers, e.g., selective catalytic reduction (SCR), non-selective catalytic reduction, selective auto-catalytic reduction, etc., as well as the use of flue gas desulfurization (FGD) scrubbers, etc. All these techniques have specific effects on the fly ash which negatively impacts the marketability of fly ash as pozzolan.
For example, there are several dry FGD scrubbing techniques that are used in coal burning operations to decrease the emissions of sulfur oxides, such as lime spray drying, duct sorbent injection, furnace sorbent injection, and circulating fluidized bed combustion. The use of any of these techniques can result in a single, commingled by-product stream consisting of coal fly ash and spent lime sorbent. The general make-up of the residual particulate matter collected following these coal burning and dry FGD scrubbing operations, often generically referred to as “spray dryer material,” are a heterogeneous combination of coal fly ash and a blend of calcium sulfate and calcium sulfite compounds.
The chemical composition of spray dryer material residues depends on the sorbent used for desulfurization and the proportion of fly ash collected with the FGD residues. The fly ash in dry FGD materials has similar particle size, particle density, and morphology to those of conventional fly ashes, but FGD materials have lower bulk densities. The difference in bulk density is due to variations in the chemical and mineralogical characteristics of the reacted and unreacted sorbent. Dry FGD materials contain higher concentrations of calcium and sulfur and lower concentrations of silicon, aluminum, and iron than fly ash.
Typically, dry FGD materials usually will not conform to the controlling specifications for pozzolans (e.g., ASTM C-618), due to the varying chemistry and glass content, the presence of high levels of calcium sulfate, and the generally heterogeneous nature of dry FGD materials. Therefore, they cannot be reliably used as pozzolan, especially for pozzolan in concrete structures.
In addition to the altered by-product particulate matter generated through the use of these various clean air strategies, air emissions from some of these pollution control techniques have in and of themselves resulted in other air pollutants. For example, at many power plants, when flue gas undergoes selective catalytic reduction of NOx, high levels of SO3 are emitted from the stack. The SO3 is visible as a “blue plume” and quickly condenses into a mist of sulfuric acid, damaging the health of humans, animals, and plant life and destroying real property.
Therefore, coal-burning operations employing combinations of certain air pollution control techniques are now being forced to mitigate the unintended consequences of their actions by further altering their operations with additional flue gas treatments to limit emissions of blue plume (SO3) aerosols or other condensable particulate matter yet to be determined and/or publicly reported in the literature.
In summary, coal burning operations have changed and will continue to change in order to comply with federally mandated and/or self-imposed limits on air emissions. These changes in coal-burning operations include, but are not limited to, the use of low-NOx burners; fuel blending/switching, flue gas conditioning with ammonia or sulfur to enhance precipitator performance; flue gas treatment to enhance the performance of NOx scrubbers; and/or FGD scrubbers to reduce the emissions of particulate matter or polluting gases, such as sulfur and nitrogen oxides.
Examples of residual chemicals and foreign particulate matter that may be deposited in or adsorbed on coal fly ash include, but are not limited to: (1) ammonia and/or SO3(solid) from flue gas conditioning; (2) ammonia from NOx reduction scrubbing and/or slip; (3) the chemical residuals from injecting hydrated lime, magnesium hydroxide, sodium bicarbonate carbonate, ammonia, sulfur, sodium bisulfate, magnacite, magnesium silicate, magnesium oxide, etc. for mitigating blue plume, i.e., SO3(gas); and, (4) mercury-laden sorbents such as activated carbon from mercury scrubbing.
The presence of any of these foreign residual chemicals and/or particulate matter will negatively impact the utilization of fly ash in general and will especially negatively affect the value of fly ash marketed as pozzolan in concrete.
The deterioration of fly ash quality referenced above negatively impacts the value, marketability and, therefore, the utilization of fly ash in the U.S. Specifically, a reduction in the pozzolanic reactivity reduces the strength-producing characteristics; excessive unburned carbon is associated with poor air-entraining characteristics; and chemical residues in the fly ash can negatively impact the marketability of fly ash as pozzolan in these and other ways, creating additional technical and aesthetic concerns.
It would be desirable to economically increase the value and utility of fly ash in the marketplace by improving those characteristics of fly ashes that have been identified by the concrete industry as being deleterious to the production of quality concrete; specifically, it would be desirable to improve pozzolanic reactivity or strength producing characteristics, air-entraining characteristics, and contamination from chemicals used for flue gas treatment.